Moisture curing hot melt adhesives composed of a urethane prepolymer having an isocyanate group in its end have been conventionally known. For example, Japanese Patent Laid-Open Gazette No. 189486/1988 and Japanese Patent Laid-Open Gazette No. 284577/1989 disclose moisture curing hot melt adhesives using a urethane prepolymer which are superior in initial adhesive strength.
Furthermore, it has been known that an organic metal compounds such as dibutyltin dilaurate or tin octate or a tertiary amine compounds such as triethylamine or triethylenediamine is added as a catalyst to this type of moisture curing hot melt adhesives.
The initial heat resistance of the above described moisture curing adhesives composed of the urethane prepolymer depends on the softening point similarly to a case of ordinary hot melt adhesives. Thus, it takes a very long period for the adhesives to exhibit practical heat resistance with progress of polymerization due to moisture curing. Consequently, an adherend must be subjected to aging for two to three days after adhesion. As a result, in the conventional adhesives disclosed in the above described prior art, the adherend must be aged for a long time, so that the productivity of a bonding operation cannot be sufficiently increased, which is uneconomical.
On the other hand, if the above described catalyst composed of organic metal compounds or the like is added, the moisture curing characteristics of the adhesives can be enhanced, thereby to make it possible to shorten a curing time period. However, if the catalyst composed of the organic metal compounds or the like is added, the thermal stability of the adhesives is significantly lowered, so that the adhesives may, in some cases, be gelled in a coating machine.
More specifically, in the conventional moisture curing hot melt adhesives composed of the urethane prepolymer, both the curing speed and the thermal stability cannot be made adequate, so that the improvements have been required.